Steel wire and method of manufacturing the same

ABSTRACT

A steel wire, 0.10-0.40 mm in diameter, obtained by subjecting a high-carbon (0.70-0.90 wt. %) steel wire material to heat treatment and wire drawing, wherein its tensile strength and test values of special repeated torsional tests satisfy a predetermined relation; and a method of manufacturing the same. A high strength steel wire which has so high a ductility as to substantially prevent the wire from being broken even during wire twisting, and which rarely encounters a decrease in the ductility even after the wire has been subjected to age hardening by heating, is obtained, and a method of manufacturing the same is economical.

TECHNICAL FIELD

The present invention relates to a steel wire used for reinforcement ofrubber articles or the like having high strength and excellent ductilityand a method of manufacturing the same.

BACKGROUND ART

In a conventional manufacturing process for a steel wire used forreinforcement of rubber articles such as steel radial tires or highpressure hoses, a high carbon steel rod containing about 0.70˜0.90% inweight of carbon is drawn to an intermediate diameter and subjected to aheat treatment and brass plating to form a steel wire material, and thenthe steel wire material is drawn to the final diameter. When the steelwire thus obtained is used for reinforcement of a rubber article, thesteel wire is embedded in non-vulcanized rubber in a form of a singlewire or a steel cord formed by a plurality of the steel wire twistedtogether, and then heated to achieve vulcanization of the rubber andadhesion between the steel wire and the rubber.

Recently, a steel wire of higher strength is strongly desired withgrowing demand for conservation of energy and natural resource. In orderto produce a steel wire of higher strength by the conventionalmanufacturing process described above, it is necessary to increase theamount of drawing performed on the steel wire material. However, whenthe amount of drawing is increased, ductility of the steel wire isdeteriorated to cause frequent wire breakage in processing or poordurability in use. And in some cases, deterioration of ductilityparticularly at the surface layer of the steel wire can be a rulingfactor on possible amount of drawing or achievable strength. Thisphenomenon is due to the fact that the drawing strain can concentratemore easily at the surface layer than can at the internal portion of thesteel wire, making the surface layer become unable to withstand furtherdrawing earlier than the internal portion. Moreover, deterioration ofductility at the surface layer can be aggravated by age hardening orpoor lubrication due to heat generated by friction with drawing die. Inorder to overcome such problems in ductility, some improvements indrawing technique have been proposed.

Among such improvements in drawing technique, one approach is to controlage hardening of steel wire by suppression of heat generation duringdrawing. For example, JP-A 8-24938 discloses a manufacturing method of ahigh tensile strength steel wire having such ductility that the steelwire can be given a large amount of torsion until it breaks when thewire is twisted to one direction wherein drawing at the final die iscarried out with control of heat generation by limitation of frictioncoefficient and application of a skin pass with reduction of 2˜11%.

Further, JP-A 8-218282 discloses a high tensile steel wire having torquereduction ratio of less than or equal to 7% in a torsion-torque test inwhich a steel wire is twisted to one direction and then twisted to theopposite direction. As a manufacturing method of such a steel wire, JP-A8-218282 also discloses a drawing method wherein (1) drawing resistanceis reduced by using dies of shorter bearing, (2) skin pass is adoptedfor the final drawing using double die, (3) dies with sintered diamondnib are used at several passes located downstream in order to reducedrawing force, and (4) temperature of lubricating fluid is maintainedlow.

However, even though a steel wire of less age hardening degree can beproduced, the above drawing methods do not give essential improvement asto the concentration of strain at the surface layer, rather, causingmore concentration of strain at the surface layer in case of applyingexcessively low reduction per die. Therefore, good ductility of a steelwire shown immediately after drawing can be largely deteriorated bypreforming in cabling or by progression of age hardening due to heatingin rubber.

Conventionally, ductility of a steel wire has been evaluated by value ofbreaking torsion which is defined as an amount of unidirectional torsionapplied to a steel wire until the steel wire is broken, or byconsidering both value of breaking torsion and form of fracture. Anotherevaluating method is adopted in JP-A 8-218282 wherein ductility of asteel wire is evaluated by torsion-torque curve obtained in a torsiontest in which a steel wire is twisted in one direction for a certainnumber of turns and then twisted in the opposite direction until thesteel wire is broken.

However, steel wires showing good results in above evaluating methods donot always maintain good ductility after preforming such as cabling orafter heat aging followed by preforming, and improvement in durabilityof rubber articles reinforced by such steel wires is not assured.

Generally, in production of a steel cord for reinforcement of rubberarticles, steel wires are preformed so as to have minimum radius ofcurvature ranging from about 10 to 150 times their diameter.Particularly, production of such steel cords listed below comprises sucha severe preforming that a steel filament is preformed so as to haveminimum radius of curvature ranging from about 10 to 60 times itsdiameter. Therefore, when a conventional steel wire is used as afilament of such steel cords, ductility is considerably deteriorated bythe severe preforming and further deteriorated largely by heating inrubber.

(1) A steel cord having so-called “open structure” comprising largelypreformed steel filaments.

(2) A steel cord comprising a steel filament preformed into a polygonalspiral shape or wavy shape.

(3) A steel cord of core and sheath structure having a core comprising asteel filament formed into a wavy shape.

Another approach for controlling the deterioration of ductilityaccompanying with increase in tensile strength is to make distributionof strain in a steel wire developed by drawing more uniform so as tocontrol deterioration of ductility in the surface layer where the strainreaches maximum. For example, JP-A 7-305285 discloses a method formanufacturing a steel wire wherein:

(1) reduction of each die used for drawing where drawing strain ε isless than 0.75 is set within a range between (22.67 ε+3)% and 29%,wherein ε=2ln(d₀/d), d₀ is diameter in mm of steel wire material beforedrawing, and d is diameter in mm of steel wire after passing the die,

(2) reduction of each die used for drawing where ε is not less than 0.75and not more than 2.25 is set within a range between 20% and 29%; and

(3) reduction of each die used for drawing where ε is more than 2.25 isset within a range between (−6.22 ε+43)% and (−5.56 ε+32.5)%.

By this method, substantial drawing strain at the surface area can becontrolled, but controlling effect on age hardening due to heatgenerated by drawing is insufficient, and economical production becomesdifficult with increasing drawing speed because of frequent wirebreakage in cabling or drawing process.

In view of above problems of prior art, it is an object of the presentinvention to provide a steel wire having such a excellent ductility thatthe steel wire hardly breaks in cabling and little deteriorates bypreforming or age hardening after preforming. And another object is toprovide a method for economically manufacturing such a steel wire.

DISCLOSURE OF THE INVENTION

After various experiments and studies, the inventors found that veryimportant points for achievement of above objects are;

(1) ductility of surface layer of a steel wire should be evaluated andregulated by a specially arranged repeated torsion test, and

(2) optimization of reduction per die at the final die is necessary aswell as uniform distribution of strain induced by drawing for economicalmanufacturing of such a steel wire.

The present invention has been done based on the important pointsmentioned above and includes following aspects in which [1]˜[4] relateto a steel wire having excellent ductility which little deteriorates bypreforming or by age hardening after preforming, and [5]˜[7] relate to amethod of manufacturing such a steel wire economically.

[1] A steel wire having a diameter ranging from 0.10 mm to 0.40 mmobtained by subjecting a high-carbon steel wire material having a carboncontent ranging from 0.70% to 0.90% in weight to heat treatment and wiredrawing, characterized in;

that tensile strength TS (N/mm²) of the steel wire satisfies followingformula

TS≧2250−1450 log D  (1)

wherein D is the diameter of the steel wire in mm and log means commonlogarithm,

and that repeated torsion value RT (turns/100D) of the steel wire, whichis defined as sum of forward twisting and reverse twisting given until acrack is formed on a steel wire in a test wherein a steel wire issubjected to a repetition of forward twisting equivalent to 3 turns per100D and reverse twisting to the original state, satisfies followingformula.

log RT≧2−0.001{TS−(2250−1450 log D)}.  (2)

[2] A steel wire having above characteristics wherein tensile strengthTS (N/mm²) satisfies following formula.

TS≧2750−1450 log D  (3)

[3] A steel wire of less concentration of strain at the surface layerhaving repeated torsion value RT not less than 60% of RT of the samesteel wire the surface layer of which has been removed by the amountequivalent to 10% of total volume.

[4] A steel wire especially suitable for reinforcement of rubberarticles and having above characteristics wherein breaking torsionvalue, which is defined as an amount of twisting to one directionsubjected to a steel wire until the steel wire is broken, is not lessthan 20 turns per 100D when the steel wire has been given such apreforming that the steel wire has minimum radius of curvature of 10 to60 times its diameter and embedded in rubber and taken out from therubber after vulcanization.

[5] A method of manufacturing a steel wire having above characteristicsby drawing a high-carbon steel wire material after heat treatment,characterized in that the drawing is carried out according to followingconditions;

{circle around (1)}reduction per die is set form (22.67 ε+3)% to 29% fordies at which ε is less than 0.75,

{circle around (2)}reduction per die is set from 20% to 29% for dies atwhich ε is not less than 0.75 and not more than 2.25,

{circle around (3)}reduction per die is set from (−5.56 ε+32.5)% to(−6.22 ε+43)% for dies at which ε is more than 2.25 except for the finaldie,

{circle around (4)}reduction per die is set from 4% to (−8.3 ε+40.6)%for the final die, and

{circle around (5)}ε at the final die is set from 3.0 to 4.3,

wherein ε is drawing strain expressed by a formula ε=2ln(d₀/d) (4), d₀is diameter of the steel wire material in mm before drawing, d isdiameter of the steel wire in mm after passing through a die, and lnmeans natural logarithm.

[6] A method of manufacturing a steel wire which enables economicalmanufacturing of super high tensile steel wire, wherein ε at the finaldie is set from 3.5 to 4.2 in the method of manufacturing a steel wiredescribed above.

[7] A method which makes above method of manufacturing a steel wire moreeffective, wherein a bending operation with tension is applied to thesteel wire drawn through the final die.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing relationship between ε and reduction per diefor pass schedule A and B as well as area of reduction per die accordingto the present invention.

FIG. 2 is a graph showing relationship between ε and reduction per diefor pass schedule C, D and B as well as area of reduction per dieaccording to the present invention.

FIG. 3 is a graph showing relationship between tensile strength andrepeated torsion value for steel wires of Examples and Comparativeexamples as well as area of repeated torsion value according to thepresent invention.

FIG. 4 is an illustration of a bending apparatus.

FIG. 5 is an illustration of an equipment used for repeated torsiontest.

PREFERRED EMBODIMENT FOR IMPLEMENTING THE INVENTION

Following is a detailed explanation of repeated torsion test adopted inthe present invention. In this test, a steel wire with its axis keptstraight is subjected to a repetition of twisting equivalent to 3 turnsper length of 100times its diameter to form a crack on the steel wire.In order to keep the axis of the steel wire straight during the test,the steel wire is stretched by small tension. The steel wire is twistedby predetermined number of turns N₀, and then returned to the originalstate by the same number of turns to the reverse direction. This cycleincluding one forward twisting and one reverse twisting is repeated toform a crack on the steel wire. Here, N₀ is a number of turns equivalentto 3 turns per length of 100 times the diameter of the steel wire andexpressed by N₀=3×(L/100D), wherein L(mm) is length of the steel wiresubjected to the twisting and D(mm) is diameter of the steel wire.

Repeated torsion value RT is sum of forward twisting and reversetwisting given until a crack is formed on the steel wire expressed byamount of turns per 100D and is calculated as follows. If a crack isformed at the time when the steel wire is turned N_(f1)(≦N₀) times tothe forward direction in the cycle next to n cycles of forward tuning ofN₀ times and reverse turning, the repeated torsion value RT iscalculated by following formula(5 a).

RT=(2nN ₀ +N _(f1))/(L/100D)  (5a)

If a crack is formed at the time when the steel wire is turnedN_(f2)(≦N₀) times to the reverse direction after forward turning of N₀times in the cycle next to n cycles of forward tuning of N₀ times andreverse turning, the repeated torsion value RT is calculated byfollowing formula(5 b).

RT={(2n+1)N ₀ +N _(f2))}/(L/100D)  (5b)

Preferable conditions for above-described repeated torsion test are asfollows.

(1) Length of steel wire subjected to twisting is about 50 mm.

(2) Tension applied to the axial direction of steel wire is about 1.0 kgwhen diameter of steel wire is not more than 0.25 mm, or about 1.5 kgwhen the diameter is more than 0.25 mm.

(3) Turning rate is about 30 turns per minute.

(4) Detection of crack formation is done by detection of acousticemission (A.E.) wave accompanying with crack formation. A.E. wave is anelastic wave emitted by release of strain energy when a solid body isdistorted or fractured. By electrical detection of A.E. using A.E.sensor, formation of micro cracks prior to breaking of steel wire can beexactly detected and accurate evaluation can be done.

In the present invention, above-described repeated torsion value isadopted for a parameter of ductility of steel wire because the inventorsdiscovered that a steel wire having high repeated torsion value not onlyhas high ductility when it is subjected to the test but also littledeteriorates in ductility by preforming such as cabling or by agehardening.

Generally, saving of ductility becomes more difficult by increasingtensile strength or diameter of steel wire. Therefore, conventionalsteel wires do not satisfy following formula (1) and (2) simultaneously.

TS≧2250−1450 log D  (1)

log RT≧2−0.001{TS−(2250−1450 log D)}  (2)

However, a steel wire according to the invention satisfies both formula(1) and (2) having high strength and excellent ductility even when thesteel wire is actually used as reinforcement of rubber articles or thelike.

As to strength of a steel wire, a steel wire having tensile strengthTS(N/mm²) satisfying following formula (1) is suitable for reinforcementof rubber articles.

TS≧2250−1450 log D  (1)

But it is preferable that following formula (6) is satisfied.

TS≧2500−1450 log D  (6)

Further, remarkable effect on weight reduction of rubber articles can beobtained when following formula (3) is satisfied.

TS≧2750−1450 log D  (3)

In order to realize such a steel wire having high repeated torsionvalue, it is desirable that ductility of surface layer of a steel wireis not far from that of internal part of the steel wire where decreasein ductility is less progressed by drawing. Comparison of ductilitybetween surface layer and internal part of a steel wire can be done bycomparison of repeated torsion value between a steel wire with itssurface layer having been removed and the same steel wire with itssurface layer not removed. It is preferable that repeated torsion valueof a steel wire with its surface layer not removed is not less than 60%of that of the same wire with its surface layer having been removed.

Because a steel wire according to the present invention is littledeteriorated in ductility even when it is aged by heating after severepreforming, it can be advantageously used as a filament of previouslyreferred steel cords in which steel filaments are severely preformed tohave minimum radius of curvature ranging from about 10 to 60 times thediameter. In this case, it is preferable to use a steel wire havingbreaking torsion value of not less than 20 turns/100D in a conventionaltorsion test when the steel wire has been given such a preforming thatthe steel wire has minimum radius of curvature of 10 to 60 times itsdiameter and embedded in rubber and taken out from the rubber aftervulcanization. Then, ductility of the steel wire in a rubber article iscertainly assured.

When a steel wire of the invention is used for reinforcement of a rubberarticle, a coating having adhesive property for rubber can be formed onits surface. As a means for formation of such a coating, conventionalmeans such as drawing a steel wire material after heat treatment andbrass-plating can be adopted.

Next, a method of manufacturing a steel wire according to the presentinvention will be explained.

In order to produce a steel wire having tensile strength TS and repeatedtorsion value RT both according to the present invention, it isessential to control concentration of drawing strain at the surfacelayer of the steel wire. In general, distribution of drawing strainbecomes more uniform and the concentration of drawing strain at thesurface of a steel wire is more relieved by using smaller die approachangle and/or larger reduction per die. However, in actual operations, itis necessary to set up drawing conditions considering processingaccuracy of die, efficiency of lubrication, breaking load of the steelwire, etc. That is, if die approach angle is set too small or reductionper die is set too large, drawing strain at the surface is ratherincreased caused by difficulty of lubrication, or frequency of wirebreakage is increased thereby making it difficult to produce a steelwire with high productivity.

Therefore, in the method of manufacturing a steel wire according to thepresent invention, drawing is carried out on a heat-treated steel wirematerial according to the following conditions to produce a steel wire;

{circle around (1)}reduction per die is set form (22.67 ε+3)% to 29% fordies at which ε is less than 0.75,

{circle around (2)}reduction per die is set from 20% to 29% for dies atwhich ε is not less than 0.75 and not more than 2.25,

{circle around (3)}reduction per die is set from (−5.56 ε+32.5)% to(−6.22 ε+43)% for dies at which ε is more than 2.25 except for the finaldie,

{circle around (4)}reduction per die is set from 4% to (−8.3 ε+40.6)%for the final die, and

{circle around (5)}ε at the final die is set from 3.0 to 4.3,

wherein ε is drawing strain expressed by a formula ε=2ln(d₀/d), d₀ isdiameter of the steel wire material before drawing, d is diameter of thesteel wire after passing through a die, and ln means natural logarithm.In other words, though drawing conditions disclosed in JP-A 7-305285 areadopted for dies except for the final die, it is necessary to setreduction of the final die within a range which is lower than thatdisclosed in the same.

The reason why reduction of the final die should be set withinabove-mentioned range is as follows. In conventional wet drawingmachines, drawing at dies except for the final die is carried out inliquid lubricant, but the steel wire having passed through the final dieis not immersed in liquid lubricant. Therefore, if reduction of thefinal die is set according to the same condition as that of diesdisposed upstream, deterioration of ductility by age hardening becomessevere because of high temperature of the steel wire having passedthrough the final die This problem is aggravated by increase of drawingspeed. In order to solve the problem, the inventors examined andinvestigated concerning the reduction of the final die and found thatthe deterioration of ductility by age hardening can be controlledkeeping concentration of drawing strain at the surface of the steel wirewithin proper degree, by setting the reduction of the final die within arange of 4% to (−8.3 ε+40.6)%. If reduction of the final die is lessthan 4%, the wire may have good ductility immediately after drawing butlargely deteriorated by age hardening when the steel wire is heatedlater. So, the lower limit is set 4%. The upper limit is set (−8.3ε+40.6)% in order to control heat generation even when flow stress isincreased by increase of ε so as to suppress damage on the surface ofthe steel wire caused by deterioration of ductility of the steel wire orpoor lubrication. Satisfying this condition, increase in drawing speedor production of a super high tensile steel wire becomes easier comparedwith prior art.

Total drawing amount, which is value of ε at the final die, should befrom 3.0 to 4.3 and is selected according to strength of the steel wireto be obtained. Particularly, this invention is suitable for productionof super high tensile steel wire which needs severe drawing with ε morethan or equal to 3.5, or more than or equal to 4.0. The upper limit 4.3is set because control of deterioration of ductility becomesinsufficient if ε exceeds 4.3. Preferable value for the upper limit is4.2.

Moreover, as a means for further improvement of ductility of a steelwire, a bending operation with tension can be adopted on a drawn steelwire so as to decrease drawing strain at the surface layer of the steelwire. This operation also decreases residual stress at the surface layerof the steel wire, and a steel wire having excellent durability as areinforcement of rubber articles can be produced. Because a steel wireaccording to the invention has sufficient ductility and is hardly brokeneven when a sever bending is given, such a bending operation can beeasily adopted.

As to geometry of dies to be used, ordinary dies used for drawing ofsteel wire materials, e.g. dies having approach angle from 8 to 12degrees and bearing length of 0.3D to 0.6D, can be used. Also, materialof dies is not limited to such as sintered diamond. And dies of cheapermaterials such as cemented carbide can be used. As to the steel wirematerial to be drawn, it is preferable to use a high carbon steel wirematerial having good uniformity produced by a preferable heat treatmentin which decarburization is controlled and uniform pearlite, containingless foreign phases such as primary cementite, primary ferrite orbainite, is formed.

Hereafter, the present invention will be explained based on someexamples.

EXAMPLE 1, 2 AND COMPARATIVE EXAMPLE 1, 2

A high carbon steel wire rod of about 5.5 mm in diameter containingabout 0.82% in weight of carbon was drawn by dry drawing until itsdiameter reached about 1.67 mm. And then, patenting and brass-platingwas done to obtain a brass-plated steel wire material. The brass-platedsteel wire material had metallic structure of nearly uniform pearliteand its tensile strength TS was about 1250N/mm² measured by tensile testaccording to JIS (Japanese Industrial Standard) G3510.

The brass-plated steel wire material was drawn to produce steel wireshaving diameter of 0.28 mm on four drawing conditions shown in Table 1which are combinations of two kinds of pass schedule and whether bendingoperation after drawing is done or not. Table 2 shows detail of two passschedules A and B, and FIG. 1 shows the relationship between ε andreduction per die of respective pass schedules. As FIG. 1 shows, passschedule A satisfies the limitation of the present invention and passschedule B is a comparative example in which reduction per die at eachdie is set lower to decrease heat generation.

In the drawing, a slip-type multi-pass wet drawing machine was used withcemented carbide dies having approach angle of about 12 degrees andbearing length of about 0.5 D. The bending operation after drawing wasdone with tension of about 2 kg by using an apparatus shown in FIG. 4 inwhich number of rollers 2 was nine, diameter of rollers 2 was 16 mm andengagement 3 was 6 mm.

TABLE 1 pass schedule bending operation Example 1 A no Example 2 A yesComparative example 1 B no Comparative example 2 B yes

TABLE 2 pass schedule A Pass schedule B hole hole die diameter reductiondiameter reduction No. (mm) ε per die (%) (mm) ε per die (%) 1 1.6300.048 4.7 1.630 0.048 4.7 2 1.550 0.149 9.6 1.570 0.123 7.2 3 1.4200.324 16.1 1.470 0.255 12.3 4 1.265 0.556 20.6 1.350 0.425 15.7 5 1.1200.799 21.6 1.230 0.612 17.0 6 0.990 1.046 21.9 1.120 0.799 17.1 7 0.8751.293 21.9 1.020 0.986 17.1 8 0.770 1.548 22.6 0.930 1.171 16.9 9 0.6801.797 22.0 0.850 1.351 16.5 10 0.600 2.047 22.1 0.770 1.548 17.9 110.530 2.295 22.0 0.700 1.739 17.4 12 0.475 2.515 19.7 0.640 1.918 16.413 0.425 2.737 19.9 0.580 2.115 17.9 14 0.385 2.935 17.9 0.530 2.29516.5 15 0.350 3.125 17.4 0.485 2.473 16.3 16 0.320 3.305 16.4 0.4452.645 15.8 17 0.295 3.467 15.0 0.410 2.809 15.1 18 0.280 3.572 9.9 0.3802.961 14.1 19 0.350 3.125 15.2 20 0.325 3.274 13.8 21 0.305 3.401 11.922 0.290 3.501 9.6 23 0.283 3.550 4.8 24 0.280 3.572 2.1

For the steel wires produced by the respective conditions, tensilestrength TS and repeated torsion value RT were measured according to thefollowing conditions.

Tensile strength TS was measured by tensile test according to JIS G3510.

Repeated torsion value RT was measured by using an apparatus shown inFIG. 5. In FIG. 5, number 6 indicates a rotating chuck which holds oneend of a steel wire 1 and is rotated around the axis of the steel wire 1by a driving means 8 which is fixed on a base 12. Number 7 indicates afixed chuck which holds the other end of the steel wire 1 so as not torotate. The fixed chuck 7 is supported on the base 12 and is movable tothe axial direction of the steel wire 1. A wire 9 carrying a weight 11for giving tension to the steel wire 1 is connected to the fixed chuck 7at the side opposite to the steel wire 1 through a pulley 10.

In the measurement of repeated torsion value RT, respective ends of thesteel wire 1 were held by the rotating chuck 6 and the fixed chuck 8 andlength of the steel wire 1 between the rotating chuck 6 and the fixedchuck 7 was adjusted so as to make the length of the steel wire to betwisted be 50 mm. The weight 11 of about 1.5 kg was used. The number ofturns N₀ equivalent to 3 turns per length of 100 times the diameter ofthe steel wire is 5.36 turns according to the formula N₀=3×(L/100D). Therotating chuck 6 was driven by the driving means 8 so that the rotatingchuck 6 made repetition of 5.36 clockwise turns and 5.36counterclockwise turns to return to the original position, therebygiving the steel wire 1 repetition of twisting equivalent to 3 turns perlength of 100 times the diameter of the steel wire. The rotating speedof the rotating chuck 6 was about 30 turns per minute.

Formation of a crack was detected by A.E. sensor 4 disposed under thesteel wire 1 as shown in FIG. 5. In order to detect A.E. waveeffectively, grease 5 was put on the A.E. sensor 4 with the steel wire 1piercing through it. The A.E. sensor used had a built-in preamplifierwith gain of about 40 dB and frequency range of 90 to 300 kHz and wasconnected to a main amplifier with gain of 60 dB through a high-passfilter of 50 kHz and a low-pass filter of 1000 kHz, and the output ofthe main amplifier was displayed on a recorder. While the output of themain amplifier caused by noise was ± several tens μV output of ± severalhundreds μV was obtained when a crack was formed so that time of crackformation was clearly determined.

The results are listed below in Table 3.

TABLE 3 tensile strength Repeated torsion value (N/mm²) (turns/100 D)Example 1 3350 57 Example 2 3346 74 Comparative example 1 3332 15Comparative example 2 3322 21

As shown in Table 3, steel wires of Example 1 and 2 had tensile strengthequivalent to that of Comparative example 1 and 2, and had remarkablyhigher repeated torsion value compared with that of Comparative example1 and 2. The steel wire of Example 2, to which a bending operation hadbeen given, showed still higher repeated torsion value compared withthat of Example 1. Relationship between tensile strength and repeatedtorsion value for each steel wire is shown in FIG. 3 accompanied withresults of Example 3 and Comparative example 3, 4 which will beexplained later. As shown in FIG. 3, steel wires of Example 1 and 2satisfy limitation of repeated torsion value according to the inventionwhile those of Comparative example 1 and 2 do not satisfy thelimitation.

Further, in order to evaluate distribution of drawing strain in thesteel wires, relationship between volume of removed surface layer andrepeated torsion value was investigated with removing the surface layerby dissolution in nitric acid solution. The results are shown in Table4. As shown in Table 4, repeated torsion values of steel wires ofExample 1 and 2 with the surface layer not removed (ratio of removedsurface layer=0 vol. %)were not less than 60% of that with the surfacelayer equivalent to 10% by volume having been removed. Among the steelwires of Examples, the steel wire of Example 2, on which a bendingoperation after drawing was performed, showed especially high value. Onthe other hand, repeated torsion values of the steel wires ofComparative example 1 and 2 with the surface layer not removed (ratio ofremoved surface layer=0 vol. %) were much lower than 60% of that withthe surface layer equivalent to 10% by volume having been removed.

TABLE 4 ratio of removed repeated torsion value (turns/100 D) surfaceComparative Comparative layer (%) Example 1 Example 2 example 1 example2 0 57  (71) 74  (91) 15  (19) 21  (28) 1 61  (75) 75  (93) 20  (25) 22 (30) 5 75  (94) 78  (96) 59  (73) 59  (80) 10 80 (100) 81 (100) 75(100) 74 (100) *Numbers in parentheses indicate relative value whereinvalue for ratio of removed surface layer = 10% is set 100 for each case.

Further, relationship between volume of removed surface layer andF.W.H.M. (Full Width Half Maximum) of X-ray diffraction peak for ferrite211 at the surface emerged after removal of surface layer wasinvestigated so as to make comparison of distribution of drawing strainin ferrite. The result is listed in Table 5. As shown in Table 5,F.W.H.M. for ferrite 211 of steel wires of Example 1 and 2 with thesurface layer not removed (ratio of removed surface layer=0 vol. %) weresmaller than those of Comparative example 1 and 2, and differenceagainst F.W.H.M. for ferrite 211 with the surface layer having beenremoved were smaller. Further, F.W.H.M. for ferrite 211 of the steelwire of Example 2, on which a bending operation after drawing wascarried out, with the surface layer not removed (ratio of removedsurface layer=0 vol. %) was still smaller than that of Example 1, anddifference against F.W.H.M. for ferrite 211 with the surface layerhaving been removed was still smaller, too. Therefore, it can beconsidered that distribution of drawing strain in ferrite was made moreuniform with less concentration of drawing strain at the surface layerowing to a manufacturing method according to the present invention andfurther improved by bending operation.

Measurement of F.W.H.M. for ferrite 211 was done according to thecondition shown in Table 6 by using a microfocus X-ray diffractometerequipped with P.S.P.C. (Position Sensitive Photo Counter) type X-raydetector. And the value of F.W.H.M. is F.W.H.M. of diffraction peakformed by K α l spectrum separated by calculation.

TABLE 5 F.W.H.M. of X-ray diffraction peak for ferrite 211 (degree)ratio of removed Comparative Comparative surface layer (%) Example 1Example 2 example 1 example 2 0 1.08 0.94 1.29 1.24 1 1.00 0.91 1.261.24 5 0.90 0.89 0.98 0.99 10 0.88 0.88 0.91 0.92

TABLE 6 target Cobalt acceleration voltage 40 kV current 100 mA diameterof collimator 100 μm measurement time 2000 seconds

Further, in order to estimate ductility in use for reinforcement ofrubber articles, breaking torsion value (amount of twisting to onedirection subjected to a steel wire until the steel wire is broken)before and after heat aging for each steel wire was measured afterforming into a wave shape having pitch of 4.5 mm and amplitude of 0.46mm. This measurement was done by using an apparatus shown in FIG. 5according to the following condition and rotating the rotating chuck 6to one direction until the steel wire was broken.

twisted length of steel wire: 50 mm

axial tension: about 1.5 kg

turning rate: about 30 turns per minute

In addition, breaking torsion value before and after heat aging for eachsteel wire without forming was measured by same way. These results areshown in Table 7. Heat aging was carried out by heating steel wires in aoven at 145° C. for 40 minutes. As shown in Table 7, repeated torsionvalues for steel wires of Comparative example 1 and 2 without formingand heat aging were equivalent to that of Example 1 and 2, however, theywere considerably reduced by either wave forming or heat aging or both,falling into less than 20 turns per 100D. On the other hand, repeatedtorsion values for steel wires of Example 1 and 2 were less reduced byeither wave forming or heat aging or both, maintaining more than 20turns per 100D. Particularly, repeated torsion value for the steel wireof Example 2, on which bending operation after drawing was performed,was little reduced by either wave forming or heat aging or both.

TABLE 7 repeated torsion value (turns/100 D) wave heat ComparativeComparative forming aging Example 1 Example 2 example 1 example 2 no no33 34 31 34 yes 30 34 11 15 yes no 27 33 3 3 yes 25 34 2 3

Further, steel cords having a construction of core formed by wavyfilaments and a sheath shown in Table 8 were produced using each kind ofsteel wires for filaments of one steel cord, and they were embedded inrubber sheets and vulcanized at 145° C. for 40 minutes. After that, thesteel cords were taken out from rubber and decomposed into separatefilaments and repeated torsion value for each filament was measured. Asa result, repeated torsion values for steel wires of Example 1 and 2were more than 20 turns per 100D while those of Comparative example 1and 2 were less than 20 turns per 100D, showing a result similar to thecase with wave forming and heat aging shown in Table 7.

TABLE 8 Forming minimum number radius of of curvature filaments Shape(mm) core 1 wave with amplitude of 0.46 mm about 4  and pitch of 4.5 mmsheath 6 spiral with amplitude of 0.92 mm about 16 and pitch of 14 mm

EXAMPLE 3, COMPARATIVE EXAMPLE 3 and 2

A high carbon steel wire rod of about 5.5 mm in diameter containingabout 0.82% in weight of carbon was drawn by dry drawing until itsdiameter reached about 1.53 mm. And then, patenting and brass-platingwas done to obtain a brass-plated steel wire material. The brass-platedsteel wire material had metallic structure of nearly uniform pearliteand its tensile strength TS was about 1250N/mm².

The brass-plated steel wire material was drawn to produce steel wireshaving diameter of 0.19 mm on three drawing conditions shown in Table 9.Table 10 shows detail of three pass schedules C, D and E, and FIG. 2shows relationship between ε and reduction per die of respective passschedules. As FIG. 2 shows, pass schedule C satisfies the limitation ofthe present invention. Pass schedule D is a Comparative example whereinreduction per die except for the final die satisfies the limitation ofthe present invention but excessively low at the final die. And passschedule E is another Comparative example wherein reduction per dieexcept for the final die satisfies the limitation of the presentinvention but excessively high at the final die.

In the drawing, a slip-type multi-pass wet drawing machine was used withcemented carbide dies having approach angle of about 9 degrees andbearing length of about 0.5 D. The bending operation after drawing wasdone with tension of about 2 kg by using an apparatus shown in FIG. 4 inwhich number of rollers 2 was twenty, diameter of rollers 2 was 12 mmand engagement 3 was about 3 mm.

TABLE 9 pass schedule bending operation Example 3 C yes Comparativeexample 3 D yes Comparative example 4 E yes

TABLE 10 pass schedule C pass schedule D pass schedule E die holediameter reduction hole diameter reduction hole diameter reduction No.(mm) ε per die (%) (mm) ε per die (%) (mm) ε per die (%) 1 1.480 0.0666.4 1.480 0.066 6.4 1.480 0.066 6.4 2 1.390 0.192 11.8 1.390 0.192 11.81.390 0.192 11.8 3 1.280 0.357 15.2 1.280 0.357 15.2 1.280 0.357 15.2 41.155 0.562 18.6 1.155 0.562 18.6 1.155 0.562 18.6 5 1.020 0.811 22.01.020 0.811 22.0 1.020 0.811 22.0 6 0.900 1.061 22.1 0.900 1.061 22.10.900 1.061 22.1 7 0.790 1.322 23.0 0.790 1.322 23.0 0.790 1.322 23.0 80.700 1.564 21.5 0.700 1.564 21.5 0.700 1.564 21.5 9 0.615 1.823 22.80.615 1.823 22.8 0.615 1.823 22.8 10 0.545 2.064 21.5 0.545 2.064 21.50.545 2.064 21.5 11 0.483 2.306 21.5 0.483 2.306 21.5 0.483 2.306 21.512 0.430 2.538 20.7 0.430 2.538 20.7 0.430 2.538 20.7 13 0.387 2.74919.0 0.387 2.749 19.0 0.387 2.749 19.0 14 0.350 2.950 18.2 0.350 2.95018.2 0.350 2.950 18.2 15 0.315 3.161 19.0 0.315 3.161 19.0 0.315 3.16119.0 16 0.285 3.361 18.1 0.285 3.361 18.1 0.285 3.361 18.1 17 0.2603.545 16.8 0.260 3.545 16.8 0.263 3.522 14.8 18 0.241 3.696 14.1 0.2403.705 14.8 0.243 3.680 14.6 19 0.224 3.843 13.6 0.223 3.852 13.7 0.2263.825 13.5 20 0.208 3.991 13.8 0.207 4.001 13.8 0.212 3.953 12.0 210.195 4.120 12.1 0.193 4.141 13.1 0.198 4.090 12.8 22 0.190 4.172 5.10.190 4.172 3.0 0.190 4.172 7.9

For the steel wires produced by the respective conditions, tensilestrength TS, repeated torsion value RT and F.W.H.M. of X-ray diffractionpeak for ferrite 211 were measured. Measuring conditions for tensilestrength TS and F.W.H.M. for ferrite 211 were the same as those adoptedfor Example 1. Repeated torsion value was measured with N₀=7.89 andusing weight 11 of about 1.0 kg, other conditions being same as thoseadopted for Example 1. The results are shown in Table 11. As shown inTable 11, steel wire of Example 3, wherein reduction per die at thefinal die was within the proper range, had remarkably higher repeatedtorsion value compared with that of Comparative example 3 and 4, withtensile strength equivalent to that of Comparative example 1 and 2.Moreover, F.W.H.M. for ferrite 211 at the surface of steel wire ofExample 3 was smaller than that of Comparative example 3 and 4.Relationship between tensile strength and repeated torsion value foreach steel wire is shown in FIG. 3 accompanied with results of Example1, 2 and Comparative example 1, 2 explained before. As shown in FIG. 3,the steel wire of Example 3 satisfies limitation of repeated torsionvalue according to the invention while those of Comparative example 3and 4 do not satisfy the limitation.

In drawing with the conditions according to Comparative example 3 and 4,some wire breakages were occurred when the wire passed the bendingrollers. But no wire breakage was occurred in drawing with the conditionaccording to Example 3.

TABLE 11 tensile repeated torsion F.W.H.M of strength value ferrite 211(N/mm²) (turns/100 D) (degree) Example 3 4050 21 1.45 Comparativeexample 3 4031 11 1.48 Comparative example 4 4078 11 1.55

INDUSTRIAL APPLICABILITY

As described above, a steel wire according to the present invention hasboth high strength and excellent ductility which is little deterioratedeven when it is subjected to a preforming and/or heat aging. Therefore,the steel wire shows excellent reinforcing effect and durability when itis used for reinforcement of rubber articles such as a filament of asteel cord for a tire. By adopting a method of manufacturing a steelwire according to the present invention, a steel wire having such aexcellent property can be manufactured economically withoutdeterioration of productivity by wire breakage of poor lubrication.

What is claimed is:
 1. A method of manufacturing a steel wire having adiameter ranging from 0.10 mm to 0.40 mm obtained by subjecting ahigh-carbon steel wire material having a carbon content ranging from0.70% to 0.90% in weight to heat treatment and wire drawing,characterized in; that tensile strength TS (N/mm²) of the steel wiresatisfies following formula, TS≧2250−1450 log D wherein D is thediameter of the steel wire in mm and log means common logarithm, andthat repeated torsion value RT (turns/100D) of the steel wire, which isdefined as sum of forward twisting and reverse twisting given until acrack is formed on a steel wire in a test wherein a steel wire issubjected to a repetition of forward twisting equivalent to 3 turns per100D and reverse twisting to the original state with the axis of thesteel wire kept straight, satisfies following formula, logRT≧2−0.001{TS−(2250−1450 log D)} which comprises a step of drawing ahigh-carbon steel wire material after heat treatment, characterized inthat the step of drawing is carried out according to followingconditions; {circle around (1)}reduction per die is set from (22.67ε+3)% to 29% for dies at which E is less than 0.75, {circle around(2)}reduction per die is set from 20% to 29% for dies at which ε is notless than 0.75 and not more than 2.25, {circle around (3)}reduction perdie is set from (−5.56 ε+32.5)% to (−6.22 ε+43)% for dies at which ε ismore than 2.25 except for the final die, {circle around (4)}reductionper die is set from 4% to (−8.3 ε+40.6)% for the final die, and {circlearound (5)}ε at the final die is set from 3.0 to 4.3, wherein ε isdrawing strain expressed by a formula ε=2ln(d₀/d), d₀ is diameter of thesteel wire material in mm before drawing, d is diameter of the steelwire in mm after passing through a die, and ln means natural logarithm.2. A method of manufacturing a steel wire comprising; a wire diameterranging from 0.10 mm to 0.40 mm obtained by subjecting a high-carbonsteel wire material having a carbon content ranging from 0.70% to 0.90%in weight to heat treatment and wire drawing, characterized in; atensile strength TS (N/mm²) of the steel wire satisfies followingformula, TS≧2250−1450 log D wherein D is the diameter of the steel wirein mm and log means common logarithm, and that repeated torsion value RT(turns/100D) of the steel wire, which is defined as sum of forwardtwisting and reverse twisting given until a crack is formed on a steelwire in a test wherein a steel wire is subjected to a repetition offorward twisting equivalent to 3 turns per 100D and reverse twisting tothe original state with the axis of the steel wire kept straight,satisfies following formula log RT≧2−0.001 {TS−(2250−1450 log D)}, saidmethod comprising the steps of heat treating drawing a high-carbon steelwire material after heat treatment, wherein the step of drawing iscarried out according to following conditions;
 1. reduction per die isset from (22.67 ε+3)% to 29% for dies at which ε is less than 0.75, 2.reduction per die is set from 20% to 29% for dies at which ε is not lessthan 0.75 and not more than 2.25,
 3. reduction per dies is set from(−5.56 ε32.5)% to (−6.22 ε+43)% for dies at which ε is more than 2.25except for the final die,
 4. reduction per die is set from 4% to (8.3ε+40.6)% for the final die, and
 5. ε at the final die is set from 3.0 to4.3, wherein ε is drawing strain expressed by a formula ε=2ln(d₀/d), d₀is diameter of the steel wire material in mm before drawing, d isdiameter of the steel wire in mm after passing through a die, and lnmeans natural logarithm.
 3. A method of manufacturing a steel wireaccording to claim 2, wherein ε at the final die is set from 3.5 to 4.2.4. A method of manufacturing a steel wire according to claim 2, whereina bending operation with tension is applied to the steel wire drawnthrough the final die.
 5. A steel wire comprising wire diameter rangingfrom 0.10 mm to 0.40 mm obtained by subjecting a high-carbon steel wirematerial having a carbon content ranging from 0.70% to 0.90% in weightto heat treatment and wire drawing, the steel wire manufactured bydrawing a high-carbon steel wire material after heat treatment, whereinthe drawing is carried out according to following condition; 1.reduction per die is set from (22.67 ε+3)% to 29% for dies at which ε isless than 0.75,
 2. reduction per die is set from 20% to 29% for dies atwhich ε is not less than 0.75 and not more than 2.25,
 3. reduction perdies is set from (−5.56 ε+32.5)% to (−6.22 ε+43)% for dies at which ε ismore than 2.25 except for the final die,
 4. reduction per die is setfrom 4% to (8.3 ε+40.6)% for the final die, and
 5. ε at the final die isset from 3.0 to 4.3, wherein ε is drawing strain expressed by a formulaε=2ln(d₀/d), d₀ is diameter of the steel wire material in mm beforedrawing, d is diameter of the steel wire in mm after passing through adie, and ln means natural logarithm and the tensile strength TS (N/mm²)of the steel wire satisfies following formula, TS≧2250−1450 logD whereinD is the diameter of the steel wire in mm and log means commonlogarithm, and that repeated torsion value RT (turns/100D) of the steelwire, which is defined as sum of forward twisting and reverse twistinggiven until a crack is formed on a steel wire in a test wherein a steelwire is subjected to a repetition of forward twisting equivalent to 3turns per 100D and reverse twisting to the original state with the axisof the steel wire kept straight, satisfies following formula,logRT≧2−0.001{TS−(2250−1450 logD)}.
 6. A steel wire according to claim5, having tensile strength TS (N/mm²) satisfying following formulaTS≧2750−1450 log D.
 7. A steel wire according to claim 6, havingrepeated torsion value RT not less that 60% of RT of the same steel wirethe surface layer of which has been removed by the amount equivalent to10% of total volume.
 8. A steel wire according to claim 5, havingbreaking torsion value, which is defined as an amount of twisting to onedirection subjected to a steel wire until the steel wire is broken, notless than 20 turns per 100D when the steel wire has been given such apreforming that the steel wire has minimum radius of curvature of 10 to60 times its diameter and embedded in rubber and taken out from therubber after vulcanization.